The present invention relates to drive tractors which are widely used in printers and other devices for handling webs such as continuous perforated paper, individual sheet paper, tape and the like, fabricated of paper, foil, laminates and other sheet-like material, all collectively referred to hereinafter as "web material".
Generally such devices employ a belt which will engage the web material either by pins or projections on the upper surface or by friction or "clamping" of the web material between the upper surface of the belt and a cover or guide thereabove. Such devices are in widespread use for the applications indicated above and are particularly burgeoning in usage for paper handling in printers for computers and word processing apparatus and in duplicating apparatus.
In Seitz U.S. Pat. No. 4,130,230 granted Dec. 19, 1978; U.S. Pat. No.4,159,794 granted July 3, 1979; U.S. Pat. No. 4,194,660 granted Mar. 25, 1980; and U.S. Pat. No. 4,315,585 granted Feb. 16, 1982, there are illustrated and described tractors of this type employing a one-piece integrally molded continuous synthetic resin belt travelling in a continuous path about a drive sprocket or pulley with which it is drivingly engaged and a cooperating arcuate surface, which is generally a second pulley or sprocket which idles. The described and illustrated tractors are mounted on a pair of spaced parallel extending shafts for use in a web feeder of a printer or like apparatus. The drive shaft is usually of rectangular cross-section and the support shaft of circular cross-section. The drive shaft is inserted in a rectangular recess within the drive sprocket or pulley to operatively engage the pulley and therefore drive the belt.
In Seitz U.S. Pat. Nos. 4,130,230, 4,159,794 and 4,194,660, the tractors are releasably clamped to the support shaft by means of a butterfly type clamping mechanism employing a locking tab to deflect a portion of the support aperture into engagement with the support shaft. Further, rigid clamping of the tractor body on the support shaft may create torque on the drive shaft during rotation thereof by the printer if there is significant error in the center-to-center spacing of the shafts. Such torque may bind the drive shaft and adversely affect the movement of the web of material.
In Seitz U.S. Pat. No. 4,315,585, the clamping mechanism for the tractor on the support shaft is provided on a journal block to permit substantial relative movement between the drive shaft and support shaft in an effort to accomodate variations in center-to-center spacing as the drive shaft is rotatably driven. However, these "accomodated" variations in the center-to-center spacing between the drive and support shafts can adversely affect the accuracy of the web driving function of the tractor. Moreover, the cam lock of the Seitz patent effects some change in the effective center of the aperture through the tractor which can compound the problem of center-to-center spacing accuracy, particularly if compensating relative movement is not provided.
It is an object of the present invention to provide a novel tractor for feeding web material which provides a simple and highly effective clamping mechanism which maintains it common center despite variations in size of the shaft received therein.
It is also an object to provide such a drive tractor which minimizes variations between the center-to-center spacing of the drive and support shafts while tolerating variation in support shaft diameter.
It is also an object to provide such a tractor wherein the novel clamp mechanism may be fabricated readily and assembled to the tractor chassis.
Another object is to provide such a tractor wherein the clamping mechanism does not impose torque on the drive shaft and is readily engaged and disengaged.
It is a further object to provide a drive tractor which may be fabricated readily and relatively economically and which will enjoy a long life in operation.